TTL Sync with SCART Cables

[Tempest]

Is there an easy way to determine if a SCART cable uses TTL Sync vs CSync?  The new Retrotink 5X Pro is coming out and it doesn't support TTL Sync, in fact it will be damaged if you try and use TTL Sync.  I have no idea if my cables that I use with my classic systems use TTL Sync or not and I can't seem to find an easy way to tell.

[carlsson]

Does either SCART cable use TTL levels? As far as I know, RGB signals nominally are 0.7 Vpp, composite video and csync up to 1.0 Vpp. On the other hand TTL levels tend to be 3-5V which explains why the Retrotink takes damage if you input too high voltages into it.

 

For comparison, if I'm making a SCART cable for my BBC Micro, which has RGB TTL levels, I need to use voltage dividers consisting of a handful resistors around 330 Ohm that bring down the ~3V levels to 1V. I would not want to feed RGB TTL directly to the TV.

 

One way to know if your system (rather than the cable itself, IMHO) for some reason outputs TTL levels through SCART which probably is non-standard, would be to use a multimeter and probe the sync pin when the system is running to read the voltage. I suppose it will fluctuate depending on the image, perhaps an entirely white image will have a higher voltage (or I'm misthinking how sync works) but if the multimeter reads well over 1V it probably is not within the 1.0 Vpp limit.

[Tempest]

34 minutes ago, carlsson said:

Does either SCART cable use TTL levels? As far as I know, RGB signals nominally are 0.7 Vpp, composite video and csync up to 1.0 Vpp. On the other hand TTL levels tend to be 3-5V which explains why the Retrotink takes damage if you input too high voltages into it.

 

For comparison, if I'm making a SCART cable for my BBC Micro, which has RGB TTL levels, I need to use voltage dividers consisting of a handful resistors around 330 Ohm that bring down the ~3V levels to 1V. I would not want to feed RGB TTL directly to the TV.

 

One way to know if your system (rather than the cable itself, IMHO) for some reason outputs TTL levels through SCART which probably is non-standard, would be to use a multimeter and probe the sync pin when the system is running to read the voltage. I suppose it will fluctuate depending on the image, perhaps an entirely white image will have a higher voltage (or I'm misthinking how sync works) but if the multimeter reads well over 1V it probably is not within the 1.0 Vpp limit.

 

Right now my SCART cables go into a SCART switch (Otaku) and that goes into a Shinybow to convert RGB to Component.  I'm not sure if this means there's no TTL Sync or not (I don't know if they accept it or not).  I don't know how common TTL cables are.  Everyone seems to say that if you have high quality cables then you'll be ok, but my cables came from a bunch of places so I don't know if they're considered 'high quality' or not.

 

So you're saying that if I turn my system on and test the voltage on the sync pin, I might be able to tell?  I do have a multimeter (one of those cheap 11 function Cen Tech ones from China), but I've never really used it before.  Can you tell me what I would have to set it to and where the probes go (one on the pin and one on ground?)

[carlsson]

I would have to verify my theory first, but if you set it to DC voltage in the low range, my idea is that between the sync pin and ground, like you mention, there would be a measurable voltage.

 

According to this sheet, the two only pins with a voltage above 1V would be pin 8 for switching between TV (tuner) and AV mode, and pin 16 for enabling RGB mode on AV. I wonder if the terminology of TTL SCART cables come from this, that a cable would apply up to 3V onto pin 16 to force the TV into RGB mode. Pin 8 to swtich from tuner to AV mode very rarely is used in a cable. The question then is if your gear chokes on voltages on either of those two pins, i.e. if it rather would like you to manually select RGB mode over composite video mode instead of that the cable/system forces it.

 

http://martin.hinner.info/vga/scart.html

 

This page a bit down the page mentions AC levels and DC levels, of which the later would be up to 2V but I'm not sure where those figures come from.

https://pinouts.ru/Audio-Video-Hardware/Scart_pinout.shtml

Edited April 30, 2021 by carlsson

[CatPix]

SCART doesn't support RGB TTL.

You need a converter or a device that support RGB TTL to use in on SCART-able devices.

 

If the system you use came with SCART natively (check if that's the case) then check the type of SCART input (A few converter devices use JP-21 that use a SCART connector but with an incompatible pinout that put the 12V in a color pin).

Or the opposite, if you got a system with a SCART cable from Japan, there's a chance it's a JP-21 cable.

Edited April 30, 2021 by CatPix

[carlsson]

Yup. But the note about up to 2V in DC mode made me curious. I suppose looking up the Retrotink more in detail might explain what they are referring to. If it is a matter of the RGB enable pin 16, I suppose disconnecting it on every cable would make the cables safe, but then you would manually have to enable RGB mode on the TV. Often that is possible through the menus but not every TV has that. For instance if you have a cheap Amiga RGB SCART cable, those sometimes lack the voltage on pin 16 meaning that a TV which lacks a menu choice to manually enable RGB only will display sync based garbage on screen and you might think your cable is broken (which it kind of is since one signal is missing). I have patched such cables by adding extra signals on the outside of the original cable, while in this case you might want to do the opposite if the converter device doesn't like to be told what type of signal it should handle.

[Tempest]

Here's a page on RGB Sync methods

 

https://www.retrorgb.com/sync.html

 

And a page on how to check what's in the SCART cable, but the info on this page has me confused:

https://www.retrorgb.com/csync.html

 

 

I should mention I'm using Euro SCART, not JPN.

[carlsson]

Aha. So we're talking about two ends here:

 

1. What the console outputs through its own connector. It is mentioned that most systems have csync that is 75 ohm (i.e. 1 Vpp) but some of the mentioned ones have TTL level sync, just like CGA, C128, BBC Micro etc. I'm surprised that the (NTSC) SNES and all the Genesis have TTL sync. In that case I would expect TTL levels on RGB too but perhaps they're having two different voltages for powering some device through the sync signal.

 

2. What the SCART end looks like. Just like we mentioned above, the spec clearly says that SCART should have 0.7 - 1.0 Vpp except for the two signals that specify which type of signal it is. Just like the page you linked to mentions, if you build a SCART cable for e.g. SNES or Genesis, you must install resistors (and sometimes capacitors) to act like voltage dividers to bring the TTL level to 1 Vpp.

 

Anyone who simply wired up the signals from any console or computer to a SCART connector without adjusting the signals was a daredevil. While it is possible that some TV's might accept TTL levels on the inputs, I don't think their internal electronics like it very much in the long end. Too high voltages on RGB would yield a saturated picture, not sure what happens if you give it a too strong sync signal.

 

So if you're making (or buying) your own cables that go directly from respective console to the converter, you would want to make sure the voltages are the right ones. Remember that RGB signals can be carried in many types of connectors besides SCART, not the least DE9 and various DIN and mini-DIN.

 

If you're making (or buying) SCART cables, those probably should already fit the standard. You could probably take it apart in either end and see e.g. in the case of a SNES cable if you're having a 330-450 ohm resistor wired into the cable or not. If so, the csync signal would be within spec. If not, someone was lazy, stupid, evil or a combination of the three when they built the cable.

[Tempest]

38 minutes ago, carlsson said:

If you're making (or buying) SCART cables, those probably should already fit the standard. You could probably take it apart in either end and see e.g. in the case of a SNES cable if you're having a 330-450 ohm resistor wired into the cable or not. If so, the csync signal would be within spec. If not, someone was lazy, stupid, evil or a combination of the three when they built the cable

That's what I'm thinking.  I doubt that my equipment is getting direct TTL Sync voltages, but who knows? 

 

Oh this is so complicated.  I never knew there were different types of syncs. 

[carlsson]

Me neither, or rather I didn't know that some systems would have different voltage levels on the RGB and sync signals. Later on I'll conduct some experiment to measure voltage levels, since I anyway am one SCART cable short for my Beeb/Electron so a good opportunity to take some measurements. I can't imagine anyone going through the hassles to make cables, either for own use or even more horribly, for professional/resale, would skip the voltage divider because "it mostly works anyway" by feeding the TV a stronger signal than it expects, but then again there are all sorts of people in this hobby so one shouldn't completely rule it out.

[CatPix]

1 hour ago, carlsson said:

Aha. So we're talking about two ends here:

 

1. What the console outputs through its own connector. It is mentioned that most systems have csync that is 75 ohm (i.e. 1 Vpp) but some of the mentioned ones have TTL level sync, just like CGA, C128, BBC Micro etc. I'm surprised that the (NTSC) SNES and all the Genesis have TTL sync. In that case I would expect TTL levels on RGB too but perhaps they're having two different voltages for powering some device through the sync signal.

 

Sega Genesis 1 / Mega Drive 1 / Master System – These consoles output TTL sync from pin 1 on the A/V connector. 

Hmm so that might be why Sega made the French versions with a different cable output.... I heard they used a "different standard" but I never had the explanation of using TL sync. Maybe the "authorities" here deemed the resistor solution "not enough" and forced Sega to put all the amplification and sync in the "RVB" box we have.

 

The TTL RVB mgiht be because except for France that made SCART mandatory on all new color TVs made after 1981, almost nobody in Europe had SCART-equipped TV around the late 80's (and never in the US) so people that would use the RGB out would have owned TTL RGB/CGA monitors that usually accept TTL and analog RVB signals with either TTL sync and H/V sync but maybe not csync.

Edited April 30, 2021 by CatPix

[carlsson]

There are multiple ways to make voltage dividers, with R/C ladders or whatever those are called, possibly with diodes and transistors too. The cheapest method is just using single resistors and rely on that you have 75 ohm impedance in the other end but just like always when there comes to electronics, the skilled people can argue for days and weeks which solution is the proper or best one. Just like with arguing about different software implementations, I suppose. The resistor method has worked for me but if French authorities wanted an even more reliable method, probably something more advanced could be constructed in the cable if they didn't want to change the console itself.

[+-^CrossBow^-]

Nearly all of the RGB SCART cables I use are wired up for Genesis 2 AV 9-pin mini din. However, the cables themselves all have caps and resistors inside them to bring the TTL levels down and so the cables are actually wired up for C-sync. HDRetrovision does the same thing only instead of using the sync wire on their cables, they actually just use the composite video output pin for sync. 

 

My guess @Tempest is that all of your cables are most likely wired for C-sync and you will be fine. What all do you have connected up that is using RGB as that might give us a better idea on what if anything you have, could possibly be connected via TTL sync?

 

[CatPix]

They changed the console in that on the motherboard, the whole RGB to composite and amplifier parts are depopulated (and the RF missing, but that's a given).

Basically they take the RGB unamplified signal straight to the A/V output and the amplification is made inside a separate box :

 

 

(you may notice that on the output part, you have "sync" but no "composite" and yes, French SMS and Megadrive I have no composite video out. Megadrive II aren't modified and output PAL. So the "SECAM" name is a technical lie ? )

 

But I agree with you, the "resistor" method is good enough for us and our hardware.

Also people at least in Germany (probably in other countries but the only few testimonies of people using SCART cables BITD I heard were from Germany) had SCART cables so most likely it was good enough outside of France.

 

[carlsson]

As a side note, the first cable I made for my BBC Micro to a Commodore 1084, I intended to use the monitor's TTL mode since it should match the voltages. However I only got a red and black image with bleeding so something must've been amiss either in the TTL circuitry of the monitor or the signals from the computer. I had to add those resistors and make the cable to work in analog RGB mode, same DE-9 connector but with the toggle switch in the other mode which worked fine. Since then, I have at least connected a 80 column C128 to the 1084 without issues so I don't know what was wrong in the first place. Perhaps the Beeb output a bit too low voltages for a real CGA monitor to accept them, but too high for a TV or analog RGB monitor to accept them.

[Tempest]

12 minutes ago, -^CrossBow^- said:

Nearly all of the RGB SCART cables I use are wired up for Genesis 2 AV 9-pin mini din. However, the cables themselves all have caps and resistors inside them to bring the TTL levels down and so the cables are actually wired up for C-sync. HDRetrovision does the same thing only instead of using the sync wire on their cables, they actually just use the composite video output pin for sync. 

 

My guess @Tempest is that all of your cables are most likely wired for C-sync and you will be fine. What all do you have connected up that is using RGB as that might give us a better idea on what if anything you have, could possibly be connected via TTL sync?

 

Everything

 

NES
SNES
N64
PS1
PS2
Genesis 2
Saturn
DC
TurboDuo
XBox

Neo Geo MVS

[CatPix]

Nothing outstanding here, so if you bought used/original SCART cable, you should be fine.

Chinese cables are often wired straight, without any resistor, capacitor and ground bridging of all the SCART pins (and that can cause problems, too).

If your cables are factory originals : they are most likely good.

If they are aftermarket old ones... probably good but can't hurt to check.

If they are recent eBay ones : check them. Pop the SCART shell and see if there are any missing resistors, caps. If they work, they work. If they don't, bridge the ground pins.

[juansolo]

Check any SCART that's had a sync-stripper added to it. Depending on where they've got the power from you can end up with TTL sync if you're not careful. Something to bear in mind if you add them to your SCART leads if using something like an Extron switch (that doesn't like sync over composite). Which incidentally can deal with TTL no problem, but you don't want any SCART outputting TTL really just incase you plug it into something that can't, so it's good practice not to.

Edited May 23, 2021 by juansolo

[DJ Clae]

If your scart cable is outputting TTL sync, then they weren't made to a proper scart standard (in other words if you own scart cables it's very unlikely unless they were made incorrectly).

 

Scart cables for consoles that output TTL sync like the Genesis must either use sync on composite or have the sync attenuated to a proper consumer TV level in the cable. Otherwise the cable was made wrong.